In enhanced mobility MOSFET device applications thick, relaxed Si1-xGex, buffer layers have been used as virtual substrates for thin, strained silicon layers to increase carrier mobility for both NMOS Welser et al., Electron mobility enhancement in strained-Si N-type metal-oxide-semiconductor transistors, 1994 IEDM Conference Proceedings, p. 373 (1994).IEEE EDL-15, #3, p. 100, (1994); Rim et al., (Rim I) Fabrication and analysis of Deep submicron strained-Si N-MOSFETs, IEEE Transactions on Electron Devices, Vol 47, 1406, (2000); and Rim et al., (Rim II) Strained Si NMOSFETs for High Performance CMOS Technology, 2001 Symposium on VLSI Technology Digest of Technical Papers, p. 59, IEEE 2001; and PMOS, Rim et al., (Rim III) Enhanced hole mobilities in surface-channel strained-Si p-MOSFETs, 1995 IEDM Conference Proceedings, p. 517 (1995); and Nayak et al., High-Mobility Strained-Si PMOSFET's [sic], IEEE Transactions on Electron Devices, Vol. 43, 1709 (1996). Compared with bulk silicon devices, enhancement in electron mobility of 70% for devices with Leff<70 nm has been reported, Rim I supra. Enhancements of up to 40% in high-field hole mobility for long-channel devices have also been found, Nayak et al., supra.
The main current technique to produce a high quality relaxed Si1-xGex buffer layer is the growth of a several μm thick, compositionally graded layer, Rim I, supra, and Nayak et al., supra. However, the density of threading dislocations is still high, e.g., >106/cm2. In addition, the integration of several μm Si1-xGex into device fabrication is not practical.
An alternative method to efficiently relax strained SiGe layers on silicon is to implant helium followed by an anneal step. Cavities formed in silicon and germanium, and their alloys, by helium implantation and annealing have been found to have a strong short-range, attractive interaction with dislocations. Introducing cavities at the SiGe/Si interface greatly enhances the relaxation rate and alters dislocation microstructures, Follstaedt et al., Cavity-dislocation interactions in Si—Ge and implications for heterostructure relaxation, Appl. Phys. Lett., 69, 2059, 1996. He implantation and subsequent annealing has been used to achieve ˜70% relaxation of 100 nm thick Si0.7Ge0.3 films with a threading dislocation density as low as 107/cm2 Luysberg et al., Effect of helium ion implantation and annealing on the relaxation behavior of pseudomorphic Si1-xGex, buffer layers on Si(100) substrates, J. Appl. Phys., vol. 92, pp 4290–4295 (2002). These films were successfully used to fabricate high performance n-type modulation-doped FETs, Herzog et al., Si/SiGe n-MODFETs on Thin SiGe Virtual Substrates Prepared by Means of He Implantation, IEEE Electron Device Letters, vol. 23, pp 485–487 (2002). Meanwhile, research has continued on the relaxation mechanisms of SiGe after He+ implantation and annealing, Christiansen et al., Strain relaxation mechanisms in He+-implanted and annealed Si1-xGex layers on Si(100) substrates, Mat. Res. Soc. Symp. Proc. Vol. 686, p. A1.6.1 (2002); Cai et al., Strain relaxation and threading dislocation density in helium-implanted and annealed Si1-xGe/Si(100) heterostructures, J. Appl. Phys., vol. 95, pp 5347–5351 (2004). Another recently published alternative method for the relaxation of SiGe films on Si is the implantation of Si+ ions. Doses of less than 2014/cm2 produce comparable results to those achieved by He+ implantation using much higher doses of 1–2×1616/cm2, Hollander et al., Strain relaxation of pseudomorphic Si1-xGex/Si (100) heterostructures after Si+ ion implantation, J. Appl. Phys., vol. 96, pp 1745–1747 (2004).
Hydrogen implantation has been found to induce exfoliation of silicon and cause shearing of macroscopic layers of silicon, Weldon et al, On the mechanism of the hydrogen-induced exfoliation of silicon, J. Va. Sc. technol. B. 15, 1065, (1997). This has been incorporated into the fabrication of high-quality silicon-on-insulator (SOI) wafers, and is known as the SmartCut™ process. In recent publications by a collaboration of S. Mantl et al. and H. Trinkaus et al., reports of the advantages of using hydrogen implantation to increase the degree of SiGe relaxation and to reduce the density of threading dislocations, Mantl et al., Strain relaxation of Epitaxial SiGe layers on Si (100)improved by hydrogen implantation, Nuclear Instruments and Methods in Physics Research B 147, 29, (1999); Trinkaus et al., Strain relaxation mechanism for hydrogen-implanted Si1-xGe/Si(100) heterostructures, Appl. Phys. Lett., 76, 3552, (2000), have been made. However, the collaboration only reported the relaxation of a SiGe having a thickness of between 2000 Å–2500 Å, with up to 22% germanium. SiGe having a 2000 Å–2500 Å thickness is not sufficient for device application. Also, a higher germanium content is desirable. Meanwhile, we have also made thicker films having higher germanium content, e.g., 30%, U.S. Pat. No. 6,746,902, to Maa et al., granted Jun. 8, 2004, for Method to Form Relaxed SiGe Layer with High Ge Content, and for reducing leakage current through proper isolation, U.S. patent application Ser. No. 10/345,551, of Hsu et al., filed Jan. 15, 2003, for Method of Reducing Si1-xGex CMOS Leakage Current; and U.S. Pat. No. 6,583,000 B1, granted Jun. 24, 2004, to Hsu et al. for Process Integration of Si1-xGex. CMOS with Si1-xGex, Relaxation After STI Formation.
In addition to the hydrogen implantation SmartCu™ process, other methods for splitting wafers for application in SOI fabrication have been proposed and developed. These methods all require the co-deposition of some other species together with hydrogen. By so doing, the hydrogen dose may be reduced, and the annealing temperature and time may also be reduced, resulting in a lower cost and a higher quality wafer. Co-implantation of boron (5e12/cm2 to 5e15/cm2) and H2+ ions (5e16/cm2) at energies to ensure overlap of the two species is described in Tong et al., Low Temperature Si Layer Splitting, Proceedings of the 1997 IEEE International SOI Conference, p. 126, (1997); Tong et al., A “smarter-cut” approach to low temperature silicon layer transfer, Applied Physics Letters, vol 72, p. 49 (1998); U.S. Pat. No. 5,877,070, to Goesele et al., granted Mar. 2, 1999, for Method for the transfer of thin layers of monocrystalline material to a desirable substrate; and Tong et al., Low dose layer splitting for SOI preparation, Proceedings of the 1998 IEEE International SOI Conference, p. 143, (1998). This method was most effective at reducing anneal temperature and times when a low temperature anneal was performed, e.g., 250° C. for 10 minutes, before any higher temperature annealing steps. Also, electrically inactive boron was more effective than electrically active boron. Boron is thought to be a good choice for co-implantation with hydrogen because it generates a large number of point defects per ion and the boron atoms may trap a cluster of hydrogen atoms. Both effects assist in the formation of platelets and microcracks, which are essential to the SmartCut™ process. The references also suggest using heavily boron-doped silicon substrates, instead of implanting the boron. Based on this, the co-implantation of boron with molecular hydrogen (H2+), followed by an anneal as a means of relaxing a SiGe film grown on a silicon substrate is disclosed in U.S. Pat. No. 6,562,703 B1, to Maa et al, granted May 13, 2003, for Molecular Hydrogen Implantation Method for Forming a Relaxed Silicon Germanium Layer with High Germanium Content. 
Helium has also been co-implanted with silicon, Agarwal et al., Efficient Production of Silicon-on-Insulator Films by Co-implantation of He+ with H+, Proceedings of the 1997 IEEE International SOI Conference, p. 44, (1997); Weldon et al., Mechanism of Silicon Exfoliation by Hydrogen Implantation and He, Li and Si Co-implantation, Proceedings of the 1997 IEEE International SOI Conference, p. 124, (1997). A dose of 1e16/cm2 helium with 7.5e15/cm2H implantation was found to be as effective as a 6e16/cm2H SmartCut™ process, reducing the total implant dose by 70%, Agarwal et al., supra.